I have found that when using a Porous Domain the CFX Solver is more stable with higher Volume Porosity values. In a Porous Domain, if I increase the Volume Porosity value and leave everything else the same, will the pressure drop be the same?
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For some cases, a low Volume Porosity value can cause convergence problem when the flow is parallel to the Fluid-Porous interface. Convergence can sometime be improved by using a higher Volume Porosity value. This won't change the calculated pressure drop if the Loss Model is formulated in terms of the Superficial Velocity. For example, if the Loss Model is based on Linear and Quadratic Resistance values, the pressure drop is expressed as: Delta P = K*delta_X * V^2 Here, K is the Quadratic Resistance Coefficient based on Superficial Velocity, delta_X is the thickness of the porous media and V is the Superficial Velocity. From this equation, we can see that the Volume Porosity will not influence the pressure drop. If the Loss Model is formulated from the True Velocity then the pressure drop will change, since the True Velocity depends on the Volume Porosity. Note that in many cases it is not necessary to use a Porous Domain; a sub-domain with a Momentum Loss model can often be used instead. |
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